1. Field of the Invention
The invention relates generally to the field of seismic exploration. More specifically, the invention relates to methods for acquiring and processing seismic data.
2. Background Art
In seismic exploration, seismic data are acquired by imparting acoustic energy into the earth near its surface, and detecting acoustic energy that is reflected from boundaries between different layers of subsurface earth formations. Acoustic energy is reflected when there is a difference in acoustic impedance between adjacent layers to a boundary. Signals representing the detected acoustic energy are interpreted to infer structures and composition of the subsurface earth structures.
In marine seismic exploration, a seismic energy source, such as an air gun or air gun array, is typically used to impart the acoustic energy into the earth. The air gun or array is actuated at a selected depth in the water while the air gun or array is towed by a vessel. The same or a different vessel tows one or more seismic sensor cables, called “streamers”, in the water. Generally the streamer extends behind the vessel along the direction in which the streamer is towed. Typically, a streamer includes a plurality of hydrophones disposed on the cable at spaced apart, known positions along the cable. Hydrophones, as is known in the art, are sensors that generate an optical or electrical signal corresponding to the pressure of the water or the time gradient (dp/dt) of pressure in the water. The vessel that tows the one or more streamers typically includes recording equipment to make a record, indexed with respect to time, of the signals generated by the hydrophones in response to the detected acoustic energy. The record of signals is processed, as previously explained, to infer structures of and compositions of the earth formations below the locations at which the seismic survey is performed.
Typically, in order to develop a more accurate representation of the earth's subsurface, data processing techniques are utilized to attenuate the affects of ghosting and water layer multiple reflections in the seismic data. Ghosting and water layer multiple reflections, arise because water has substantially different acoustic impedance from the air above the water surface, and water typically has a substantially different acoustic impedance from the earth formations at the bottom of the water (or sea floor).
Ghosting and water layer multiples can be understood as follows. When the air gun or air gun array is actuated, the downwardly radiating acoustic energy passes through the sea floor and into the subsurface earth formations. Some of the acoustic energy is reflected at subsurface acoustic impedance boundaries between layers of the earth formations, as previously explained. Reflected acoustic energy travels generally upwardly, and is ultimately detected by the seismic sensors (hydrophones) on the one or more streamers. After the reflected energy reaches the streamers, however, it continues to travel upwardly until it reaches the water surface. The water surface has nearly complete reflectivity (reflection coefficient equal to unity) with respect to the upwardly traveling acoustic energy. Therefore, nearly all the upwardly traveling acoustic energy will reflect from the water surface, and travel downwardly once again. The water-surface reflected acoustic energy will also be shifted in phase by about 180 degrees from the upwardly traveling incident acoustic energy. The surface-reflected, downwardly traveling acoustic energy is commonly known as a “ghost” signal. The ghost signal causes a distinct “notch”, or attenuation of the energy within a limited frequency range, in the acoustic energy detected by the hydrophones. The notch is centered about a frequency in the detected acoustic signal related to the selected depth at which the streamer is disposed, as is well known in the art.
The downwardly traveling acoustic energy reflected from the water surface, as well as acoustic energy emanating directly from the seismic energy source, may reflect from the water bottom and travel upwardly, where it is detected by the hydrophones. This same upwardly traveling acoustic energy will also reflect from the water surface, once again traveling downwardly. Acoustic energy may thus reflect from both the water surface and water bottom a number of times before it is attenuated, resulting in so-called water layer reverberations. Such reverberations can have substantial amplitude within the total detected acoustic energy, masking the acoustic energy that is reflected from subsurface layer boundaries, and thus making it more difficult to infer subsurface structures and compositions from seismic data.
It is known in the art to provide a so-called “dual sensor” cable for detecting acoustic (seismic) signals for certain types of marine seismic surveys. One such cable is known as an “ocean bottom cable” (OBC) and includes a plurality of hydrophones located at spaced apart positions along the cable, and a plurality of substantially collocated geophones on the cable. The geophones are responsive to the velocity of motion of the medium to which the geophones are coupled. Typically, for OBCs the medium is the water bottom or sea floor. Using signals acquired using dual sensor cables enables particularly useful forms of seismic data processing. Such forms of seismic data processing generally make use of the fact that the ghost signal is substantially opposite in phase to the acoustic energy traveling upwardly after reflection from subsurface layer boundaries. The opposite phase of the ghost reflection manifests itself in the measured signals by having opposite sign or polarity in the ghost signal as compared with upwardly traveling acoustic energy in the signals measured by the hydrophones. Because geophones are sensitive to the direction of signal propagation as well as the phase, the polarity of the signal detected by the geophones will be the same for the upwardly traveling acoustic energy and for the downwardly traveling acoustic energy.
The foregoing relationship between polarities of upgoing and downgoing acoustic energy has led to a number of “deghosting” and water layer effect attenuation techniques. One such technique is described in U.S. Pat. No. 4,486,865 issued to Ruehle. Pairs of detectors each comprise a geophone and a hydrophone. A filter is applied to the output of at least one of the geophone or hydrophone in each pair so that the frequency content of the filtered signal is adjusted. The adjustment to the frequency content is such that when the filtered signal is combined with the signal from the other sensor, the ghost reflections cancel.
U.S. Pat. No. 5,621,700 issued to Moldoveanu also discloses using at least one pair of sensors in a method for attenuating ghosts and water layer reverberations.
U.S. Pat. No. 4,935,903 issued to Sanders et al. discloses a method for reducing the effects of water layer reverberations which includes measuring pressure at vertically spaced apart depths, or by measuring pressure and particle motion using sensor pairs. The method includes enhancing primary reflection data for use in pre-stack processing by adding ghost data.
U.S. Pat. No. 4,979,150 issued to Barr discloses a method for marine seismic exploration in which output of substantially collocated hydrophones and geophones are subjected to a scale factor. It is said that the collocated hydrophones and geophones can be positioned at the sea floor or above the sea floor.
Most techniques known in the art for deghosting and multiple attenuation are intended for use with OBCs. It is desirable to be able to deghost and attenuate water layer multiples in signals acquired using streamer-type cables towed by a vessel. Using streamers is particularly desirable because moving a streamer from one location to another is much less time consuming and much less difficult than moving an OBC. Further, it is desirable to have a method for deghosting and multiple attenuation which is relatively insensitive to the water depth at which the streamer is positioned (towed), is relatively insensitive to undulations in the water surface, and for which knowing the energy source “wavelet” (acoustic signature) beforehand is unnecessary.
Other patents which discloses methods related to the subject matter of the present invention include the following:
J. T. Fokkema and P. M. van den Berg, Method and System for Deghosting, U.S. Pat. No. 6,477,470, issued Nov. 5, 2002; J. T. Fokkema and P. M. van den Berg, Method and System for Evaluating Quality of Deghosted Seismic Data, U.S. Pat. No. 6,654,694 issued Nov. 25, 2003; and J. T. Fokkema and P. M. van den Berg, Method and System for Deghosting, U.S. Pat. No. 6,747,913, issued Jun. 8, 2004.